Image forming apparatus with power supply for charging nip forming member and rotary fixing member

Abstract

The image forming apparatus includes a fixing apparatus capable of restraining offset of a toner in multifarious types of sheets and in a variety of environments at low costs. The fixing apparatus has a rotary fixing member and a nip forming member which forms a fixing nip portion by coming into contact with the rotary member. A recording material bearing an unfixed toner image is conveyed while being pinched by the nip portion. The fixing apparatus fixes a toner image onto the recording material. The fixing apparatus further has a bias applying means which applies a bias to the nip forming member. The bias applying means, before the recording material reaches the nip portion, applies a first bias to the nip forming member so that a surface of a rotary member is charged with electricity, and applies a second bias to the nip forming member while the recording material is conveyed by the nip portion.

What is claimed is: 
     
       1. An image forming apparatus which forms a toner image on a recording material, comprising:
 an image forming unit which forms an unfixed toner image on the recording material; 
 a rotary fixing member which is brought into contact with the unfixed toner image; 
 a nip forming member which forms a nip portion by coming into contact with said rotary fixing member, the nip portion pinching and conveying the recording material; and 
 a power supply which applies a voltage to said nip forming member, 
 wherein before a leading edge of the recording material enters the nip portion, said power supply applies a first voltage to said nip forming member during the time it takes for said rotary fixing member to complete one rotation or more, the first voltage having the same polarity as a charge polarity of the toner and being equal to or larger than a discharge starting voltage for starting a discharge to said rotary fixing member from said nip forming member, so that a surface of said rotary fixing member in a circumferential direction of said rotary fixing member is charged with the same polarity as the charge polarity of the toner. 
 
     
     
       2. An image forming apparatus according to  claim 1 , wherein when the recording material passes through the nip portion, said power supply applies a second voltage to said nip forming member, the second voltage having the same polarity as the charge polarity of the toner and an absolute value smaller than an absolute value of the first voltage. 
     
     
       3. An image forming apparatus according to  claim 2 , wherein the second voltage is smaller than the discharge starting voltage. 
     
     
       4. An image forming apparatus according to  claim 2 , wherein when the recording material passes through the nip portion, an absolute value of a surface potential of said rotary fixing member is larger than an absolute value of a surface potential of said nip forming member. 
     
     
       5. An image forming apparatus according to  claim 1 , wherein said rotary fixing member includes an endless belt, and wherein said image forming apparatus further comprises a heater which contacts an inner peripheral surface of said endless belt and forms the nip portion together with said nip forming member via said endless belt. 
     
     
       6. An image forming apparatus which forms a toner image on a recording material, comprising:
 an image forming unit which forms an unfixed toner image on the recording material; 
 a rotary fixing member which is brought into contact with the unfixed toner image; 
 a nip forming member which forms a nip portion by coming into contact with said rotary fixing member, the nip portion pinching and conveying the recording material; and 
 a power supply which applies a voltage to said rotary fixing member, 
 wherein before a leading edge of the recording material enters the nip portion, said power supply applies a first voltage to said rotary fixing member during the time it takes for said nip forming member to complete one rotation or more, the first voltage having the same polarity as a charge polarity of the toner and being equal to or larger than a discharge starting voltage for starting a discharge to said nip forming member from said rotary fixing member, so that a whole surface of said nip forming member in a circumferential direction of said nip forming member is charged with the same polarity as the charge polarity of the toner. 
 
     
     
       7. An image forming apparatus according to  claim 6 , wherein when the recording material passes through the nip portion, said power supply applies a second voltage to said rotary fixing member, the second voltage having the same polarity as the charge polarity of the toner and an absolute value smaller than an absolute value of the first voltage. 
     
     
       8. An image forming apparatus according to  claim 7 , wherein the second voltage is smaller than the discharge starting voltage. 
     
     
       9. An image forming apparatus according to  claim 7 , wherein when the recording material passes through the nip portion, an absolute value of a surface potential of said rotary fixing member is larger than an absolute value of a surface potential of said nip forming member. 
     
     
       10. An image forming apparatus according to  claim 6 , wherein said rotary fixing member includes an endless belt, and wherein said image forming apparatus further comprises a heater which contacts an inner peripheral surface of said endless belt and forms the nip portion together with said nip forming member via said endless belt.